1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, a copying machine or the like.
2. Description of the Related Prior Art
In a conventional image forming apparatus, there are employed various mechanisms: for example, a mechanism in which an optical sensor for detecting a sheet placement position is disposed in the inside of a sheet tray, a mechanism in which a switch or an actuator using an optical sensor is disposed upwardly of a sheet placement position, and the like.
Specifically, in the conventional mechanism in which the optical sensor for detecting the sheet placement position is disposed in the inside of the sheet tray, since it is necessary to detect a plurality of sheets, a plurality of optical sensors are necessary. On the other hand, in the conventional mechanism using the switch or actuator, because the switch or actuator comes into contact with the sheet or sheet tray, there is raised a problem that the switch or actuator can be damaged.
A conventional image forming apparatus which is capable of detecting a plurality of abnormal conditions in a sheet supply part thereof using a small number of optical sensors is disclosed in Japanese Patent Publication No. 6-144642 of Heisei.
According to the image forming apparatus of Japanese Patent Publication No. 6-144642 of Heisei, there is disclosed a structure in which presence or absence of a sheet within a sheet tray is detected by an optical sensor disposed near a sheet feed roller, and, when there exists a sheet which is pushed out of the sheet tray in the non-recording time, this condition is detected as defective sheet supply; that is, the two kinds of abnormal conditions (presence/absence of sheet, and defective sheet supply) can be detected by a pair of optical sensors.
However, in the structure disclosed in Japanese Patent Publication No. 6-144642 of Heisei, the optical axis of the optical sensor is disposed in the sheet feed-out opening of the sheet tray and, therefore, for example, when the rear end portion of a sheet placed in the sheet tray is folded, even though the folded portion of the sheet is deviated from the optical axis of the optical sensor, the optical sensor is not able to detect the folded portion of the sheet, which results in the jammed sheets or in the poor sheet printing.